1. Field of the Invention
The present invention discloses a novel method for the substantial elimination of the major cause of biofouling in recirculating water systems, in particular, those systems recirculating water for cooling purposes, such as, for example, water cooling towers, air conditioning systems, and the like.
2. Description of the Art
Biological fouling of circulating cooling water systems is a common problem resulting from excessive growth and development of different types of simple life forms (e.g., microorganisms such as algae, bacteria and fungi.) Circulating cooling water systems are excellent places for the incubation and growth of biological organisms because such systems contain nutrients (typically organic contamination) from air drawn into the system and from organic materials naturally occurring in the water. In addition, the water temperature in cooling towers is warm enough to provide an ideal incubation environment. Biological growth can foul pipelines, increase water circulating costs, cause and/or accelerate corrosion of metal, attack wood, and substantially reduce heat transfer thereby contributing to decreased efficiency of the cooling tower system.
Common forms of microorganisms found in a cooling tower system include algae, slime-forming fungi and bacteria, wood destroying organisms and sulfate reducing organisms along with many other forms of bacteria which may have little or no effect on cooling tower efficiency.
It is generally desirable that a biocide meet the following criteria:
(1) wide kill spectrum--the agent should be effective against a wide variety of microorganisms, such as, for example, algae, bacteria, fungi, mold and other aquatic organisms;
(2) fast rate of kill;
(3) low cost;
(4) useful in wide pH ranges;
(5) non-corrosive to metals and wood;
(6) compatible with commonly used cooling water treatment chemicals such as scale inhibitors and corrosion inhibitors:
(7) unaffected by organic contaminants or nitrogen compounds in the water recirculating system;
(8) ease in handling and application; and
(9) capable of obtaining appropriate federal and state governmental agency approval.
Biocides can be divided into two basic classifications: non-oxidizing and oxidizing biocides. In general, the non-oxidizing biocides function primarily by altering the permeability of the cell walls of the microorganisms and interfering with their biological processes. Common non-oxidizing biocides include organo-sulfur compounds, quaternary ammonium salts, chlorinated phenolics and heavy metal compounds.
Oxidizing biocides cause irreversible oxidation/hydrolysis of protein group in the microorganism and of the polysaccharides that bind the microorganisms to the surfaces of the cooling tower equipment. The result of this process is a loss of normal enzyme activity and cell death.
Oxidizing biocides heretofore proposed for cooling water use include:
(1) Chlorine;
(2) Bromine;
(3) Chloroisocyanurates;
(4) Chlorine dioxide;
(5) Hypochlorites;
(6) Bromine chloride and bromine-chlorine mixtures;
(7) 1-bromo-3-chloro-5,5-dimethylhydantoin ("BCDMH")
Each of these common biocides will be briefly discussed.
(1) Chlorine
Chlorine is probably the most common biocide in use for cooling tower treatment. It is generally an excellent algicide and bactericide although some strains of bacteria can develop chemical resistance to chlorine. Often chlorine must be used in a shock treatment system to provide good biocide performance. Gas chlorination equipment is costly and generally requires a relatively large capital investment. Normal use levels must be dramatically increased to maintain effectiveness when cooling water has become contaminated with hydrocarbons, ammonia and organic material.
Excessive chlorine concentrations have an adverse effect on cooling tower wood. Chlorine also tends to lower pH by its formation of HCl in water. Chlorine becomes less effective as a biocide above about pH 8.0-8.5 and becomes corrosive below about pH 6.5. Chlorine is a heavy greenish-yellowish gas with a suffocating odor. It requires special heavy and cumbersome steel cylinders under pressure to be transported. The recent industrial concern about industrial leaks and safety have made handling of chlorine cylinders even more suspect.
(2) Bromine
Liquid bromine has also been used in the treatment of biofouled cooling towers. However, bromine has not received widespread commercial acceptance, apparently because of handling difficulties and the cost of bromination equipment, as well as its low solubility in H.sub.2 O. (3.43 g/100 g water @ 30.degree. C.)
______________________________________ Vapor Pressure of Bromine: .degree.C. (mm Hg) ______________________________________ 20 173 25 214 30 264 ______________________________________
(3) Chlorine Dioxide
Chlorine dioxide is usually classified as an oxidizing biocide although its kill mechanism is not oxidative. It is more effective at a higher pH or in nitrogen or organic contaminated systems than chlorine. Because it is an unstable compound, it is usually generated on-site with special equipment. It is also more expensive than chlorine.
(4) Chloroisocyanurates
Chloroisocyanurates are easily handled powdered compounds which hydrolyze in water to slowly release chlorine and cyanuric acid. However, they suffer all the drawbacks of chlorine in pH effectiveness ranges and present potential corrosion problems.
(5) Hypochlorites
Sodium and calcium hypochlorites function in much the same manner as chlorine gas but in an easier to handle form. However, hypochlorites have all the disadvantages of chlorine plus a higher cost. These products also tend to increase pH by the formation of metal hydroxides and additional reagents must be added to achieve control. There is also a concern of quick gassing when product is added to water. Liquid hypochlorites also suffer from quick decay rates of active agent because they are unstable.
(6) Bromine Chloride and Bromine-chlorine Mixtures
Bromine chloride, available only as a liquid under pressure, has found some favor as a biocide. It hydrolyzes completely in dilute aqueous solutions to hypobromous acid (HOBr) and hydrochloric acid (HCL). The hypobromous acid is an effective, potent biocide for algae and bacteria. Bromine chloride has generally not been promoted for use on industrial recirculating cooling towers because of the high cost for feed equipment and accessories. Mixtures of bromines and chlorine have also been investigated as biocides. Such mixtures may be applied as a liquid/gas mixture or in the form of sodium hypochlorite and sodium hypobromite. It has been reported that a bromine/chlorine mixture displays greater biocide activity than bromine or chlorine alone. The costs of handling, as well as the safety issues involved with such mixtures have prohibited their widespread use.
(7) BCDMH
BCDMH serves as an excellent biocide in recirculating cooling towers and other water systems. Its solid form makes it easy to handle and clean-up after, and its predominate use of bromine chemistry makes it very efficient where chlorine is not. However, there are certain conditions where BCDMH has limitations. The product has low solubility in cold water, requires specialized feeding equipment to optimize product dissolution and requires high pressure or expensive options to the equipment for large applications.